Offshore platforms used in the recovery of mineral resources such as oil and gas from below the sea bed must be removed and appropriately disposed of when the wells serviced by the platforms run dry. The platforms are anchored to the ocean floor by piles which are hollow casings or pipes that are contained within the platform legs and driven into the sea bed, to depths up to and beyond 160 feet. Although the piles may vary in size, a common diameter is 42 inches. The platforms draw oil up through conductors which are made of several hollow casings of different diameters stacked within each other and extending to various depths below the sea bed.
U.S. regulations require that when removing an offshore platform, that the piles and conductors must be cut at least 16 feet below the mud line so that no projections are left which could pose a navigational hazard. However, when the piles are driven into the ocean floor, the contents of the sea bed are captured within the pile thereby creating an obstruction or "plug" that extends from the ocean floor to the bottom of the pile. The outer wall of the pile is also enclosed in mud. In order to remove the piles 16 or more feet below the mud line, therefore, it is first necessary to eliminate the mud plug or mud around the pile to at least that depth. It is common, however, to remove mud to a depth of 30 feet to create a cushion of airspace to work within in case a first attempt to remove the pile is unsuccessful and another attempt must be made higher up due to accumulated debris from the first attempt.
Current methods used to remove the mud plugs include lowering a hollow pipe the length of the casing to the mud plug. Such pipes typically have a ring of nozzles at the end nearest the mud plug that are coupled to a jet pump to deliver high volumes of water to the top of the mud plug and have an air line within the pipe to lift the mud or slurry upward through the pipe to the surface, where it is expelled into open water. Current methods for removing mud from around the pile include sending divers down to literally dig the pile out. The platforms may then be removed via available means, for example, with explosives or mechanical cutters driven from the surface, or by the method disclosed in a co-pending U.S. patent application entitled "Method and Apparatus for Cutting Metal Casings With an Ultra High Pressure Abrasive Fluid Jet."
While currently available methods provide acceptable results, it is believed that it is possible to create an improved method and system for removing mud plugs or mud from around casings that is easier to use and more efficient.